The present invention relates generally to semiconductor image sensors and more particularly to an image sensor structure with enhanced photosensitivity.
In the 1950s, researchers found that a fully charged reverse-biased P-N junction would discharge at a rate proportional to the light it received. This is because photons (light) can assist electrons and holes overcome the energy gap. These electron-hole pairs incur discharging current when they recombine after their lifetimes expire. As a result, the P-N junctions can be used as a solid-state image sensor to replace vacuum tube devices with photomultipliers to detect radiations. A CMOS image sensor, which comprises arrays of active MOS image sensor cells that are produced in a CMOS process, is one of the typical image sensing devices that utilize the photoconductive characteristics of the reverse-biased P-N junction structure.
FIG. 1 illustrates a conventional 3T CMOS image sensor cell 100 which comprises a P-N junction diode 110, a reset NMOS transistor 120, an amplifier NMOS transistor 130 and a row select NMOS transistor 140. The P-N junction diode 110, which serves as a photo-detector, and the reset NMOS transistor 120 are serially connect between a power supply VRST and a ground (GND). When the reset MOS transistor 120 is turned on by the RST signal, the P-N junction diode 110 is effectively connected to the VRST and reverse biased. When light shines on the P-N junction diode 110, an additional combination current generated by photon created electron-hole pairs cause a voltage drop at node VC. The voltage drop is then amplified by the NMOS transistor 130, which has a power supply VDD. However, the VDD is traditionally tied to the VRST. The row select NMOS transistor 140 is coupled between the amplifier NMOS transistor 130 and a column line (COL). A row line (ROW) is connected to a gate of the NMOS transistor 140. Therefore, the row select NMOS transistor 140 is a switch that allows a signal row of an array the CMOS image sensor cells 100 to be read by a read-out circuit. The aforementioned combination current flowing through the P-N junction diode 110 is proportional to the intensity of the light, therefore the read-out voltage and/or current at the COL is also proportional to the intensity of the light.
FIG. 2 is a cross-sectional view of such CMOS image sensor 100 forming an array of cells 200 in a semiconductor substrate 210. The P-N junction diode 110 and NMOS transistors 120, 130 and 140 are formed in the substrate 210. A passivation layer 220 is applied on the substrate 210. Then a planarization layer 230 is processed on top of the passivation layer 220 to make the semiconductor surface flat, for subsequent applications of a color filter 240, a spacer 250 and micro-lenses 260. All these layers 220 through 260 merely pass the light to the substrate 210, where the P-N junction diode 110 is the only device that has the photoconductive effect. Therefore, the conventional CMOS image sensor cell 200 has only mediocre optical sensitivity and signal-to-noise ratio.
As such, what is needed is an improved image sensor cell structure that has enhanced photosensitivity.